Water soluble polymers can be prepared in several forms. For instance, it is known in the art that water-soluble polymers can be dispersed into the oil phase of water-in-oil emulsions. Conventional emulsion polymerization techniques generally involve either:
(1) forming a water-in-oil emulsion of an aqueous solution of at least one water-soluble ethylenically unsaturated monomer in an inert hydrophobic liquid organic dispersion medium and polymerizing said monomer or monomers in said dispersion medium to form a polymer emulsion; or
(2) forming a water soluble polymer and then emulsifying the polymer solids into an emulsion having a hydrophobic liquid as the continuous phase.
In such polymerization methods, virtually any water soluble monomer containing a CH.sub.2 .dbd.C functional group can be polymerized and dispersed into a water-in-oil system. Thus, anionic, cationic, nonionic and amphoteric emulsion polymers can be prepared by conventional emulsion polymerization techniques.
Water-in-oil emulsions are generally comprised of three primary components including: (1) a hydrophobic phase; (2) an aqueous phase; and (3) a surfactant system. The hydrophobic phase generally comprises from about 5 to about 75%, by weight, of the emulsion. Any inert hydrophobic liquid can be used as the hydrophobic phase. Preferred hydrophobic liquids, for example, are selected from the group consisting of benzene, xylene, toluene, mineral oils, kerosene, napthas, petroleums, and blends of aromatic and aliphatic hydrocarbons containing 4 or greater carbon atoms. Particularly preferred hydrophobic liquids are those selected from the group consisting of a narrow fraction of a branch-chain hydrocarbon sold by Witco Chemical Company under the tradename Kensol 61.RTM.and branch-chain isoparafinic hydrocarbons sold by Exxon under the tradename Isopar.RTM..
The aqueous phase generally comprises from about 25 to about 95%, by weight, of the emulsion. This phase comprises the desired ethylenically unsaturated monomer or monomers and water. Additionally, this phase may contain an effective amount of a chelating agent, such as a sodium salt of ethylene diamine tetracetic acid (EDTA) or nitrilotriacetate (NTA), buffers, chain transfer agents or other additives. The monomer(s) to be polymerized generally comprise about 10 to about 60%, by weight, of the aqueous phase, with the balance being primarily water.
The surfactant system generally comprises about 0.5 to about 20%, by weight, of the emulsion. Any surfactant system which effectively disperses an aqueous phase into a hydrophobic phase can be used. See, for example, U.S. Pat. No. 3,826,771, which discloses the use of sorbitan monooleate as water-in-oil emulsifying agent; U.S. Pat. No. 3,278,506, which discloses the use of ethylene oxide condensates of fatty acid amides as emulsifiers; U.S. Pat. No. 3,284,393, which discloses the use of hexadecyl sodium pthalate, sorbitan monooleate, sorbitan monostearate, cetyl or stearyl sodium pthalate and metal soaps as water-in-oil emulsifiers; and U.S. Pat. No. 4,024,097, which discloses the use of surfactant systems comprising an oil-soluble alkanolamide, such as Witcamide.RTM. 511, which is commercially available from Witco Chemical Company, and one or more co-emulsifiers selected from the group consisting of unesterified dialkanol fatty amides, quaternized ammonium salts of fatty tertiary amines, salts of fatty tertiary amines, alkaline metal salts of fatty acids and alkyl or alkylaryl sulfates or sulfonates.
U.S. Pat. Nos. 4,672,090 and 4,772,659 disclose a surfactant system which comprises 25 to 85%, by weight, an oil-soluble alkanolamide; 5 to 35%, by weight, a polyoxyethylene derivative of a sorbitan ester; and 0 to 50%, by weight, sorbitan monooleate. Additionally, other surfactants, such as sorbitan monostearate, may be used in combination with these primary surfactants.
Emulsion polymers are generally produced by first mixing the surfactant system with the hydrophobic phase. The aqueous phase is then typically prepared by adding the monomer(s) to water in the desired concentration. Additionally, a chelant, such as a sodium salt of EDTA, may be added to the aqueous solution and the pH of the aqueous phase may be adjusted to between about 3.0 and 10.0, depending on the monomer(s) used. The aqueous monomer phase is then added to the mix containing the hydrophobic liquid and the surfactant system. The surfactant system enables the aqueous phase, which contains the monomer or monomers to be polymerized, to be emulsified into the hydrophobic phase. Polymerization is then carried out in the presence of a free radical generating catalyst, and the temperature of the reaction mixture is generally maintained between about 5.degree. and about 100.degree. C., preferably between about 20.degree. and about 50.degree. C., resulting in water-in-oil emulsion polymer. Any type of free radical initiator can be used, including, for example persulfate and azo initiators. High energy irradiation can also be used to initiate polymerization.
The following patents provide further background relative to the manufacture of emulsion polymers.
U.S. Pat. No. 3,041,318 discloses emulsifying an aqueous dispersion of a polymer prepared from a compound containing a CH.sub.2 .dbd.C group into a water-in-oil emulsion, and then precipitating the polymer therefrom as discrete particles.
U.S. Pat. No. 3,284,393 discloses a water-in-oil emulsion polymerization process wherein a water-soluble monomer is emulsified in an oil phase and polymerized therein, resulting in a polymeric latex which is then separated from the reaction medium.
U.S. Pat. Nos. 3,624,019 (reissued and reexamined as B1 Re 28,474) and 3,734,873 (reissued and reexamine as B1 Re 28,576) disclose the preparation of water-in-oil emulsions of vinyl addition polymers using various surfactants, particularly low HLB surfactants, as emulsifiers. The polymers are dispersed into emulsion by means of agitation. Thus, polymers which are powders or "lumplike agglomerates" are comminuted and dispersed into the oil phase using mechanical energy. The specification at column 3 states that "The invention contemplates using emulsions containing between 5-75 percent by weight with preferred emulsion having a polymer concentration within the range of 10-45% by weight. In some cases the starting emulsions are converted to suspensions due to the nature and the amount of the polymer present therein." Though purely theoretical polymer concentrations of up to 75% are recited, the activity range found in the examples is 23-37%.
U.S. Pat. No. 3,826,771 discloses "high solids" water-in-oil emulsions which have a polymer concentration between about 20 and 50%, by weight, based on emulsion weight. This reference discloses that the method of the U.S. Pat. No. 3,041,318 cannot be used to prepare emulsions containing 20 to 50%, by weight, of a vinyl addition polymer having a molecular weight in excess of 1,000,000 because aqueous solution cannot be prepared containing high enough concentrations of the polymer.
U.S. Pat. No. 3,888,945 discloses the use of azeotropic distillation to remove water from a suspension of an aqueous solution of a polymer in a non-polar suspending medium.
U.S. Pat. No. 3,997,492 discloses stable water-in-oil emulsion polymers which have polymer concentration between 10 and 50%, by weight, of the emulsion.
U.S. Pat. No. 4,021,399 discloses a method for concentrating a water-in-oil emulsion polymer by distilling under vacuum while maintaining the water-to-oil phase ratio substantially equal to that of the starting latex. Though this method is stated to be useful in preparing emulsions containing up to 70%, by weight, polymer, the examples are limited to .ltoreq.48% active polymer.
U.S. Pat. No. 4,035,317 discloses a method for preparing free-flowing solid polymer particles from water-in-oil emulsions. A hot gas stream is used for drying.
U.S. Pat. No. 4,052,353 discloses the treatment of water-in-oil emulsions via evaporation to reduce the water content thereof and to produce a polymer stable to coagulation or agglomeration. The use of falling-film evaporation is not disclosed or suggested.
U.S. Pat. No. 4,035,347 discloses a method for preparing substantially dry polymers from water-in-oil emulsions using a thin film drying technique.
U.S. Pat. No. 4,528,321 discloses the distillation of dispersions to remove water and other volatile components. Theoretical polymer concentrations as high as 85%, by weight, are disclosed. However, no examples are given showing greater than 57% polymer.
UK Pat. No. 1,482,515 discloses water-in-oil emulsions which theoretically contain up to 63% active polymer.
U.S. Pat. No. 4,506,062 discloses a reverse phase suspension polymerization process and the use of azeotropic distillation to prepare polymer dispersions in oil which theoretically contain up to 70% polymer solids. In various examples the polymer dispersions of the earlier examples are centrifuged to assess particle size range and shape. A centrifuge is used in a standard compaction test; this reference does not, however, disclose the use of a centrifuge to concentrate an emulsion. The process is believed to degrade molecular weight and solubility properties.
U.S. Pat. No. 5,155,156 discloses a process wherein the water in a polymer latex containing water and an emulsifier is evaporated, followed by separation of the dried polymer from the emulsifier via filtration or centrifugation, multiple washing steps and redispersion of the polymer into a second oil. The water content of the polymer after drying is "most preferably about 3% to 8%" and this reference does not disclose the use of falling-film evaporation to dehydrate polymer emulsions. Though 85% active polymer compositions are claimed, the maximum solids concentration obtained in the examples is 68%.
In summary, the inventors believe that the references discussed above provide a comprehensive overview of the emulsion polymerization art, but that they, alone or in combination, do not disclose or remotely suggest the instant invention. While certain of these references suggest, in purely theoretical terms, emulsions containing in excess of 70% active polymer, stable emulsions at such concentrations are simply not commercially available. This is verified by the examples of the above references, which only show the preparation of emulsion polymers containing less than about 70% active polymer.
Commercial emulsion polymer compositions contain substantial portions of water and are much less active. For example, U.S. Pat. No. 4,035,317 states that: "Two problems still existed, however, in the first place, the cost of transporting the emulsion is materially higher than if the polymer per se were being transported. Since the emulsions contain, on the average, only about 25-30% polymer, 70-75% of the material shipped is useless oil and water. Secondly, the emulsions can present a stability problem when exposed to temperatures below their freezing point."
Water soluble polymers containing in excess of about 60% solids are currently only commercially available in dry or gel form. Dry polymers are generally prepared by drying conventional solution polymers, and gels are simply high viscosity polymer solutions which can not be transported as liquids. They are generally adherent, tough, rubbery substances which retain their shape when placed on flat surfaces. Gels are not generally used in gel form but instead as dilute solutions of the polymer. Dilution methods oftentimes involve mechanical steps which result in the degradation of certain properties of the polymers, such as molecular weight.
To obtain polymers having high molecular weights and good water solubility properties, it is necessary to prevent crosslinking reactions during polymerization to the extent possible. In this respect, it is desirable to prepare polymers under relatively mild conditions, e.g., conducting polymerization reactions using relatively low monomer concentrations.
Unfortunately, aqueous solution polymerization techniques which use low monomer concentrations require that the resulting solution polymers be dried to prepare powdered polymers, thus resulting in increased manufacturing costs and degradation of polymer properties such as molecular weight and solubility.
Though there is a long felt need in industry for stable, high molecular weight, soluble, easily handled polymer delivery systems containing in excess of 70% active polymer, this need has not been met because of the art's inability to efficiently concentrate liquid polymer compositions to a stable form having a high polymer content without substantially degrading the resulting high solids polymers.
Also, while dry polymers provide high activity, handling and make-down problems remain. A major benefit of the instant polymer compositions is that the economic advantages of dry polymers compared to emulsion and solution polymers can be realized along with the additional advantages associated with emulsion polymerization techniques, such as that water soluble polymers of high molecular weight are prepared in a pourable, stable liquid form. This invention advances the art of polymer preparation and delivery in that stable, pourable, high molecular weight, water soluble polymer compositions containing in excess of about 60%, preferably in excess of 70%, active polymer are now available.
It is also noteworthy that no prior art reference known to the inventors discloses or suggests: 1) the use of falling-film evaporation to dehydrate emulsion polymers, particularly to water contents of less than 3%, based on total composition weight, and 2) the use of a separation means such as a centrifuge to further concentrate a dehydrated polymer/emulsifying surfactant/hydrophobic liquid composition produced via a falling-film evaporator into a high solids end use product. This further supports the novelty of the instant invention, which relates to the use of a falling-film evaporation and additional steps to dehydrate and concentrate starting water-in-oil emulsions, dispersions or suspensions, thereby producing non-aqueous polymer/emulsifying surfactant/hydrophobic liquid compositions containing less than about 5% water, preferably less than about 3% water, and greater than 60%, preferably greater than 70%, polymer solids.